A common characteristic of all bioactive implants is formation of a hydroxy-carbonate apatite (HCA) layer on their surface when implanted. The substance of human bone is essentially hydroxyapitite (Ca.sub.5 [(OH)--(PO.sub.4).sub.3 ] that is permeated with collagen. The regeneration of bone substances proceeds from mineral hydroxyapatite. It is believed that this substance acts as a point of attachment for bone substance. Starting from hydroxyapatite nuclei, a substantially complete bone is thus regenerated and built up. A bioactive material undergoes chemical reactions at the interface between tissues and the implant material. The surface reactions lead to bonding of tissues at the interface. The level of bioactivity in bioactive ceramics is dependent on composition and structure. Similarly, the mechanical properties of glass-ceramics, among others variables, depend on volume fraction, grain size, crystal phase and shape of crystals. Accordingly, the composition and method of manufacture of bioactive ceramics can have great effect on the resultant properties of the ceramic.
The processing of glass-ceramic materials has classically been viewed as a two-stage event consisting of nucleation and growth stages. The nucleation kinetics for glass-ceramic systems are described by nucleation rate curves. The most common method for generation of nucleation rate curves is the two-stage method. The first step is the production of a matrix of nucleated samples by heat treating the parent glass at varying heat treatment temperatures and heat treatment times. This matrix of samples is given a second heat treatment of sufficient time and temperature that the nuclei generated in the first step are grown to a microscopically observable size.
The growth kinetics are described by growth rate curves, which are also determined with the two-stage method. During in the first step all samples are nucleated with the same thermal treatment. In the second step, the nuclei are grown with varying heat treatment times and heat treatment temperatures. By optical microscopy measurements, the crystal size evolution for a specific temperature is determined as function of heat treatment times and a growth rate is calculated.
Previous bioactive ceramics have proven unsatisfactory because they fail to combine both the advantages of superior physical strength and a high level of bioactivity. For example, one disadvantage of known glass ceramics is their relatively low tendency to form nuclei. Furthermore, the number of nuclei formed per unit of volume is very difficult to control technologically since it is dependent on numerous factors. See U.S. Pat. No. 3,981,736 (Column 2, lines 58-68).
WO 93/17976 discloses bioactive glasses or glass ceramics, in granular form, as bone substitutes. They are described as useful for filling e.g. craniofacial bone defects, sinus lift, alveolar augmentation and bone cysts. The glasses disclosed have the following composition:
______________________________________ SiO.sub.2 53.0-62.0% Na.sub.2 O 15.0-30% CaO 10.0-25.0% P.sub.2 O.sub.5 0.0-8.0% B.sub.2 O.sub.3 0.0-3.0% Al.sub.2 O.sub.3 0.0-1.5% ______________________________________
The process disclosed for making the ceramic material includes mixing the raw materials and heating them at about 1350.degree. C. for several hours. After this the melt is poured on a graphite plate and annealed at about the glass transition temperature of the glass for one to several hours. The glass is then crushed and ceramics are produced by heating the base glass to about 650-1000.degree. C. for several hours. These compositions have the disadvantage of low bioactivity.
Accordingly, it is an object of the present invention to provide a bioactive ceramic with both high bioactivity and controlled mechanical properties similar to natural bone.
It is further an object of the present invention to provide a method for preparing such superior bioactive ceramics.